1. Field of the Invention
The present invention relates to a plasma etching apparatus, and particularly to a dry etching apparatus (ECR etching apparatus) which employs electron cyclotron resonance.
2. Description of the Related Art
FIG. 18 is a schematic drawing of a conventional plasma etching apparatus. In the drawing, a stage 3 for mounting and holding a sample, e.g., a semiconductor wafer 2, thereon is disposed in a reaction chamber 1. A gas inlet tube 4 for introducing reactive gas into the reaction chamber 1 is provided on the upper side of the reaction chamber 1. Microwave generating means, e.g., a microwave power source 5, for generating microwaves is provided outside the reaction chamber 1. Microwave energy with a predetermined frequency generated by the microwave power source 5 is introduced into the reaction chamber 1 through a waveguide and a quartz window 7. In addition, a coil 8 serving as magnetic field generating means is provided on the outer periphery of the quartz window 7 of the reaction chamber 1. The coil 8 causes the vertical application of a magnetic field with a predetermined magnetic flux density to a surface of the semiconductor wafer 2 mounted on the stage 3. The magnetic field generating means may comprise either a coil or a permanent magnet. An exhaust port 9 is also provided on the lower side of the reaction chamber 1, and exhaust means (not shown) such as a vacuum pump or the like which is connected to the exhaust port 9 functions to evacuate the reaction chamber 1 and keep it at a predetermined degree of vacuum.
In the conventional plasma etching apparatus configured as described above, when the semiconductor wafer 2 is etched, the reaction chamber 1 is first evacuated, and the reaction gas such as a halogen gas or the like is then introduced from the gas inlet tube 4 while the reaction chamber 1 is evacuated to keep the interior of the reaction chamber 1 at a predetermined pressure. Microwave energy is then generated by the microwave power source 5 and is introduced into the reaction chamber 1 through the waveguide 6 and the quartz window 7, and a magnetic field is applied to the reaction chamber 1 by the coil 8. The resonance between the magnetic field and the microwave energy causes electrons performing a cyclotron motion to absorb energy so that a high-density plasma is generated by collision of the electrons with the reactive gas. The generated plasma is carried to the semiconductor wafer 2 along the lines of magnetic force generated by the coil 8 to etch the semiconductor wafer 2.
A so-called ion sheath electric field is generated on the surface of the semiconductor wafer 2 irradiated with the plasma in the direction perpendicular to the semiconductor wafer 2 by the potential difference between the plasma potential and a floating potential. Cations (referred to as "ions" hereinafter) present in the plasma are accelerated by the ion sheath electric field and are thus applied to the surface of the semiconductor wafer 2 with good rectilinearity, thereby forming a fine pattern on the surface of the semiconductor wafer 2.
Since the ions in the plasma are accelerated by the ion sheath electric field, as described above, the ions are applied to the semiconductor wafer 2 with a uniform directional property. However, the electrons in the plasma are decelerated by the ion sheath electric field and are thus applied to the semiconductor wafer 2 without a directional property.
This state is further described in detail below with reference to FIGS. 19 and 20. FIGS. 19 and 20 are enlarged sectional views of the surface of the semiconductor wafer 2 and show the behavior of the ions and the electrons during fine pattern etching of the semiconductor wafer 2 using a plasma etching apparatus. In FIG. 19, an SiO.sub.2 film 10, an Si film 11 and a resist pattern 12 are formed in turn on the surface of the semiconductor wafer 2 so that etching is performed using as a mask the resist pattern 12. Since both the ions and electrons are applied to the surface of the resist pattern 12 as etching proceeds, electrical neutrality is maintained. Since the ions enter a micro-pattern 13 in a direction perpendicular to the surface of the semiconductor wafer 2, the ions reach the bottom 15 of the micro-pattern 13 without colliding with the side wall 14 of the micro-pattern 13. On the other hand, since the electrons have no directional property, as described above, the electrons are applied to the side wall 14 of the micro-pattern 13 and thus do not easily reach the bottom 15 of the micro-pattern 13.
In this case, when a conductive film such as the Si film 11 is etched, as shown in FIG. 19, the ions applied to the micro-pattern bottom 15 and the electrons applied to the micro-pattern side wall 14 recombine in the film to neutralize it, thereby maintaining electrical neutrality. However, when an insulating film such as the SiO.sub.2 film 10 is exposed as etching proceeds, as shown in FIG. 20, the ions applied to the micro-pattern bottom 15 are not neutralized by the electrons applied to the micro-pattern side wall 14, and thus the micro-pattern bottom 15 is positively charged. On the other hand, the micro-pattern side wall 14 is negatively charged up by the electrons applied thereto. The orbit of the ions applied to the micro-pattern bottom 15 is thus bent by repulsion of the positive charge in the micro-structure bottom 15 that is positively charged and by attraction of the micro-pattern side wall 14 that is negatively charged. As a result, the ions locally enter the interface between the Si film 11 and the SiO.sub.2 film 10 to form a so-called notch.
In order to prevent the occurrence of such a notch, the plasma etching apparatus shown in FIG. 21 is generally used. In the plasma etching apparatus shown in FIG. 21, a RF (high frequency) power source 17 is connected to a stage 3 through an impedance matching unit 16. The RF power source 17 causes the application of a RF bias voltage to a semiconductor wafer 2 so that ions can be increased in energy by acceleration and applied to the semiconductor wafer 2. In the apparatus, since the orbit of the ions is slightly bent by the charging because of the high ion energy, a notch does not easily occur. However, the high ion energy causes etching of the SiO.sub.2 film 10 as a base film, thereby causing the problem of decreasing the etching selectivity.
As shown in FIG. 18, the lines B of magnetic force produced by a coil 8 diverge from the coil 8 at the center and they are not perpendicular to the semiconductor wafer 2 at the circumference of the surface of the semiconductor wafer 2. Although ions are applied to the semiconductor wafer 2 with good rectilinearity because the ions are accelerated by the ion sheath electric field, since the ions are produced by the collision of electrons with the reactive gas, the ions have a tendency to move along the lines B of magnetic force so as to follow the electrons. There is thus the problem that the ions are not vertically applied to the semiconductor wafer 2 at the circumference of the semiconductor wafer 2, and thus anisotropic etching cannot be sufficiently performed.
In addition, the lines of magnetic force B are sparse at the circumference of the semiconductor wafer 2, as compared with the central portion thereof. There is also the problem that since a difference in the plasma density at the central portion and the circumference of the semiconductor wafer 2 causes a difference in the etching speeds of the two portions, the uniformity of etching deteriorates.
The aforementioned plasma etching apparatus thus has the problem of producing a notch due to the local charging in the above-described micropattern, thereby deteriorating the anisotropy of etching.
There is also the problem that if the anisotropy of etching is increased by increasing the ion energy, the etching selectivity is decreased, and it is thus difficult to perform etching with the anisotropy and the etching selectivity, both of which are improved.
The apparatus further has the problem that since the lines of magnetic force diverge from the coil at the center, anisotropic etching cannot be sufficiently effected at the circumference of the semiconductor wafer 2, thereby deteriorating the uniformity of etching.